phenomena of emission, ponderable matter is seen to give birth to
waves which are transmitted by the ether, and by the phenomena of
absorption it is proved that these waves disappear and excite
modifications in the interior of the material bodies which receive
them. We here catch in operation actual reciprocal actions and
reactions between the ether and matter. If we could thoroughly
comprehend these actions, we should no doubt be in a position to fill
up the gap which separates the two regions separately conquered by
physical science.

In recent years numerous researches have supplied valuable materials
which ought to be utilized by those endeavouring to construct a theory
of radiation. We are, perhaps, still ill informed as to the phenomena
of luminescence in which undulations are produced in a complex manner,
as in the case of a stick of moist phosphorus which is luminescent in
the dark, or in that of a fluorescent screen. But we are very well
acquainted with emission or absorption by incandescence, where the
only transformation is that of calorific into radiating energy, or
_vice versa_. It is in this case alone that can be correctly applied
the celebrated demonstration by which Kirchhoff established, by
considerations borrowed from thermodynamics, the proportional
relations between the power of emission and that of absorption.

In treating of the measurement of temperature, I have already pointed
out the experiments of Professors Lummer and Pringsheim and the
theoretical researches of Stephan and Professor Wien. We may consider
that at the present day the laws of the radiation of dark bodies are
tolerably well known, and, in particular, the manner in which each
elementary radiation increases with the temperature. A few doubts,
however, subsist with respect to the law of the distribution of energy
in the spectrum. In the case of real and solid bodies the results are
naturally less simple than in that of dark bodies. One side of the
question has been specially studied on account of its great practical
interest, that is to say, the fact that the relation of the luminous
energy to the total amount radiated by a body varies with the nature
of this last; and the knowledge of the conditions under which this
relation becomes most considerable led to the discovery of
incandescent lighting by gas in the Auer-Welsbach mantle, and to the
substitution for the carbon thread in the electric light bulb of a
filament of osmium